1. Field of the Invention
The present invention relates to a zinc oxide-based sputtering target.
2. Description of the Related Art
As a transparent conductive film for use in, for example, electronic devices, indium tin oxide (ITO) and the like have been widely used for many years. However, due to recent price increase of rare metals such as indium, rare-metal alternatives are strongly desired. Accordingly, attempts have been actively made in recent years to use less expensive zinc oxide (ZnO) for producing transparent conductive films, but ZnO is not yet extensively used because there are various problems, for example, the resistance is not sufficiently lowered, and a desired electrical conductivity cannot be obtained. Therefore, a zinc oxide film with higher transparency and electrical conductivity is desired as a transparent conductive material that substitutes for ITO.
Industrially, transparent conductive films are mainly produced by sputtering. However, when forming a zinc oxide-based transparent conductive film by sputtering, abnormal electrical discharge called arcing may occur. Since arcing deteriorates the quality of zinc oxide film, impairs transparency and electrical conductivity, and also damages the underlying functional layer, it is important to suppress arcing during sputtering. Moreover, in order to achieve high productivity, a high film formation rate is also required.
A sputtering target having a decreased sintered grain diameter to address the arcing problem is known. For example, Patent Document 1 (JP 4894293B) discloses an ITO or AZO (aluminum-doped zinc oxide) sintered body that is sintered by electromagnetic wave heating and that has a thickness of 10 mm or greater, a sintered grain diameter of 0.5 μm to 2 μm, and an average sintered density of the entire sintered body of 99% or greater in terms of relative density.
It is also known that a highly uniform film can be obtained by sputtering with a sputtering target composed of (002) plane-oriented zinc oxide. For example, Patent Document 2 (JP 3301755B) discloses an electrically conductive zinc oxide sintered body having an average sintered grain diameter of 1 μm to 10 μm, a volume resistivity of less than 10 Ω·cm, a density of 4.0 g/cm3 or greater and less than 5.0 g/cm3, and (002) crystal orientation greater than (101) crystal orientation. Patent Document 3 (JP H6-88218A) discloses a zinc oxide-based sintered body having (002) crystal orientation greater than (101) crystal orientation and a density of 4.5 g/cm3 or greater. However, the object of Patent Documents 2 and 3 is to form a low-resistance film or to form a film with a small resistance distribution on a low-temperature substrate, and is not to address the arcing and film formation rate problems.
On the other hand, as a technique for producing an oriented sintered body, it is known to use a plate-like raw material powder with a particle diameter of several micrometers and to array plate-like particles by pressing or the like (for example, Patent Document 4 (JP 2009-215629A)).